These efforts cover both basic and applied aspects of mycoplasmas and related wall-free prokaryotes, including their neurotoxins, antigens and other biological factors involved in virulence, their immunological interrelationships, and their possible role in human disease or disease of uncertain etiology. Current projects of interest concern the characterization and serological interrelationships of an expanding group of helical mycoplasmas (spiroplasmas) being isolated from plants and a variety of insects, especially ticks. Recent studies have confirmed that many of these new organisms possess overt pathogenicity for vertebrates (embryonated chicken eggs and suckling rats), suggesting that they may represent important pathogens for man. Further sero-epidemiological studies in many seem warranted. Recent collaborative studies on the acholeplasmas (non-sterol-requiring mycoplasmas) isolated from plant and animal sources suggest some alteration in the concept that these organisms are parasitic. The occurrence of free-living acholeplasmas in plants, some species of which had previously only been found in vertebrates, raises important questions about vector transmission and other host-parasite relationships of these organisms.